Cleaning the surface of a semiconductor is indispensable in manufacturing a thin-film transistor (TFT) and the like from a semiconductor. As cleaning methods, in the case of silicon (Si), for example, in addition to a processing method for semiconductor substrates that employs a hydrogen peroxide solution (hereinafter referred to as the RCA processing method) developed by W. Kern et al. in 1970, chemical etching that processes the surfaces with diluted hydrofluoric acid (HF/H2O) and aqueous solution of ammonium fluoride (NH4F/HF/H2O) are used as known techniques.
However, while etching the silicon surface with ammonium fluoride (NH4F) aqueous solution is effective for removing natural oxide films, further improvement is required in removal of contaminants or in particle elimination measures. Also, while progress from a step is the dominant approach in etching of a plane direction (111), etching also progresses from some terraces such that the terrace portion affected by this etching becomes rough. That is, due to the plane direction dependency of conventional chemical etching techniques, obtaining a smooth surface at the atomic level (few nm level) is difficult, and therefore a uniform and flat surface cannot be expected from a polysilicon and an amorphous silicon with non-uniform crystal surfaces.
As art relating to the present invention, the present inventors have found that a highly-uniform quality silicon dioxide (SiO2) based oxide coating can be formed on a substrate surface using only a so-called chemical action of chemicals by immersing the silicon substrate in an oxidizing solution, for example, azeotropic nitric acid (concentration of 68%, boiling point of 120.7° C.), and have proposed to use this technique for gate insulators of TFT and the like (for example, the patent document 1).
Further, the present inventors have proposed a technique in which an oxide coating is formed on the surface of a polysilicon layer having a projection of as long as 100 nm in an azeotropic nitric acid and then the oxide coating is etched away such that the projection is reduced (patent document 2).